Exhaustive Guide to Biological Immunity in Plants and Humans

Overview of Immunity and Pathogen Resistance

Immunity is defined as the body's ability, functioning through the immune system, to resist pathogens. This resistance is achieved through two primary strategies: preventing the entry of pathogens into the living organism's body or attacking and destroying foreign bodies and pathogens once they have successfully entered. Pathogenic threats include various organisms such as insects, fungi, viruses, protozoa, and bacteria.

The immune system comprises two integrated branches: Innate Immunity and Acquired Immunity. These systems function in close cooperation and coordination. Innate immunity is fundamentally essential for the successful operation of acquired immunity, and conversely, the mechanisms of the acquired system support and enhance innate responses. This interdependent relationship ensures a comprehensive defense against biological threats.

Structural Immunity in Plants

Plants defend themselves against pathogens through structural and biochemical pathways. Structural immunity involves physical mechanisms that utilize the plant's own structures. These are divided into pre-existing structural defenses and induced structural defenses that form only after infection occurs.

Pre-existing structural defenses include the epidermal cells and the cell wall. The epidermis of green stems and leaves is often covered with a waxy layer of cutin. This layer prevents water from settling on the plant surface, thereby denying fungi and bacteria the moist environment they require to thrive. Additionally, the epidermis may feature hairs or thorns to prevent the plant from being eaten by grazing animals. The cell wall provides additional support and protection for all plant cells. It is composed mainly of cellulose and can be thickened with additional cellulose, lignin, suberin, or cutin, making it physically difficult for pathogens to penetrate. Cutin specifically plays a triple role: providing physiological support by preventing water loss, offering structural support, and acting as a component of structural immunity.

Induced structural defenses are mechanisms triggered by the presence of a pathogen. These include the formation of cork ($Phellem$), which acts as an external barrier to protect the plant and is re-formed if the outer stem layer is cut or torn to prevent microbial entry. The formation of tyloses involves the overgrowth of adjacent parenchymal cells into xylem vessels to obstruct the spread of pathogens. If a vessel is partially blocked, some water passage remains, but if it is totally blocked, no water passages occur, though pathogen spread is successfully halted. Plants may also deposit gums to capture microbes and prevent their entry; examples include leguminous plants such as $Acacia\,nilotica$ trees. Cellular immune structures include the swelling of cell walls and surrounding fungal mycelium with an insulator cover to prevent transmission between cells. Finally, the hypersensitive response is a drastic measure where the plant kills its own infected tissues to isolate and eliminate the pathogen.

Biochemical Immunity in Plants

Biochemical immunity involves the plant responding to infection by secreting chemical substances. These substances may be present before infection or formed as a direct consequence of it. Receptors play a critical role in recognizing the presence of pathogens and stimulating the plant's innate immune system. These receptors are found both before and after infection.

Antimicrobial chemicals include phenols and glycosides, which are toxic compounds that kill pathogenic organisms like bacteria or inhibit their growth. Non-protein amino acids, such as Canavanine and Cephalosporins, do not enter the structure of plant proteins but act as toxic chemical compounds against pathogens. These chemicals have a direct effect on pathogens and aid in the removal of infected tissues.

Antimicrobial proteins, including detoxifying enzymes, have an indirect effect on pathogens. Rather than attacking the organism directly, they interact with the toxins produced by pathogens to invalidate their toxicity. This process effectively converts toxic substances into non-toxic ones.

The Human Lymphatic System and Primary Lymphoid Organs

The human immune system is centered around the lymphatic system, which consists of lymph, lymph vessels, and lymphatic organs. These organs are classified as primary or secondary. Primary lymphoid organs are responsible for the production, maturation, and differentiation of lymphocytes. These include the red bone marrow and the thymus gland.

Red bone marrow is located in flat bones (such as the skull, vertebral column, ribs, sternum, shoulder, and pelvis) and in the heads of long bones (such as the femur, tibia, and humerus). It is responsible for the production of enucleated red blood cells ($RBCs$), platelets, and all types of white blood cells ($WBCs$). It is the site for the maturation of B-cells and Natural Killer ($NK$) cells, which arise from lymphoid stem cells.

The thymus gland is located on the trachea, above the heart and behind the sternum. It secretes the hormone Thymosin. Thymosin is unique because its site of secretion is the same as its site of action, though it does not act on the cells that secrete it. The thymus is the site where T-lymphocytes mature and differentiate. While the thymus is large in children, it decreases in size with age, leading to a reduction in Thymosin hormone levels in the blood of older individuals.

Secondary Lymphatic Organs and Tissues

Secondary lymphoid organs serve as sites for the storage of immune cells and the interception of pathogens. The spleen is a small, dark red, lymphoid organ located on the upper left side of the abdominal cavity. Approximately the size of a human palm, it acts as a filter and a graveyard for senescent (old) red blood cells. It contains many macrophages that engulf and disintegrate old $RBCs$, recycling iron back to the liver or bone marrow. It also houses various lymphocytes.

Tonsils are two lymphoid glands located in the rear portion of both sides of the mouth. They pick up foreign bodies entering with food or air to provide an early warning and protection system. Peyer's patches are small nodes of lymphoid cells found in the mucous membrane lining the lower part of the small intestine. They play a role in the immune response against pathogens entering the digestive tract.

Lymph nodes are scattered along the network of lymphatic vessels throughout the body, specifically under the armpits, on the sides of the neck, in the upper thigh, and near internal body organs. They range in size from a pinhead to a small bean. Internally, they are divided into pockets filled with B-cells, T-cells, and macrophages. Their main functions are to purify the lymph and store $WBCs$. Lymph enters through afferent vessels and exits through efferent vessels. Valves within the node ensure lymph does not flow in the opposite direction.

Specialized White Blood Cells (WBCs)

White blood cells are categorized based on the presence of granules in their cytoplasm. Granulocytes include Neutrophils, Eosinophils, and Basophils. These cells typically remain in the blood for hours to days and are distinguished by the shape of their nucleus and the color of their granules when stained. They fight infections, especially bacterial ones, through phagocytosis and by using granules to disintegrate pathogens.

Neutrophils are the first responders in bacterial infections. They have a multi-lobed nucleus ($3-5$ lobes) and neutral-staining granules. Eosinophils have a bi-lobed "headphone-shaped" nucleus and acidic-staining (red-pink) granules. Basophils have a bi-lobed nucleus with unequal lobes and basic-staining (dark blue) granules. Mast cells, which are non-granulocytes (or agranulocytes), and basophils secrete large quantities of histamine during inflammation.

Agranulocytes include Monocytes and Lymphocytes. Monocytes are the largest $WBCs$ and possess a single core cell; they can change into macrophages to engulf pathogens and present antigens on their surface. Lymphocytes include B-cells, T-cells, and NK-cells. B-cells ($10\%-15\%$ of lymphocytes) identify microbes, bind to them, and produce antibodies. NK-cells ($5\%-10\%$ of lymphocytes) secrete perforins to destroy virus-infected cells, carcinogenic cells, and transplanted organs.

T-lymphocytes ($80\%$ of all lymphocytes) are further divided into three types: T-helper ($T_H$), which activate other T-cells and stimulate B-cells to produce antibodies; T-cytotoxic ($T_C$), which attack cancer cells, transplanted organs, and virus-infected cells; and T-suppressor ($T_S$), which regulate the immune response and inhibit T and B cells after the pathogen is eliminated.

Assisting Chemical Substances in Immunity

Several chemical substances assist and cooperate with the immune system's specialized mechanisms. Chemokines act as recruiters, drawing phagocytic cells to the site of infection to reduce microbial reproduction and spreading. Interleukins function like a wireless communication system, mediating signals between different immune cells and the body's systems.

Complements are a group of different proteins and enzymes found in the blood. After antibodies conjugate with antigens, complements are activated to lyse the antigen on the surface of the microbe and dissolve its contents, making it easier for phagocytes to engulf the pathogen. Interferons are proteins produced and secreted by tissues already infected by viruses. They are not specific to a certain virus. They bind to neighboring healthy cells to induce the production of enzymes that inhibit the replication enzymes of the virus, thereby preventing the virus from spreading.

Antibodies and the Three Lines of Defense

Antibodies, or immunoglobulins ($Ig$), are specific Y-shaped proteins that circulate in the blood and lymph. They consist of two pairs of polypeptide chains: two long heavy chains and two short light chains, held together by disulfide bonds. Each antibody has a variable region, where the antigen-binding site is located, and a constant region. The specificity of an antibody is determined by the conformation of amino acids at the antigen-binding site, following a "lock and key" model. There are five main types: $IgM$ (a pentamer, the first formed), $IgA$ (found in breast milk and mucus), $IgG$ (crosses the placenta), $IgE$ (involved in allergy), and $IgD$.

Human defense is organized into three lines. The first line of defense consists of natural barriers like skin (with a tough horny layer), cerumen (ear wax), tears (containing lysozymes), saliva, mucus in the respiratory tract (moved by cilia), and acidic gastric juice ($HCl$ in the stomach). These barriers prevent pathogens from entering the body.

The second line of defense is an internal system that acts if the first line is breached. This includes the inflammatory response, where mast cells and basophils release histamine. Histamine causes maximum dilation of blood vessels and increases the permeability of capillaries, leading to tissue swelling and allowing $WBCs$ and interferons to reach the site of injury. NK-cells and macrophages also participate in this non-specific response.

The third line of defense is the acquired (specific) immunity. It involves two mechanisms: Humoral (Antibody-mediated) immunity and Cellular (Cell-mediated) immunity. Humoral immunity depends on B-cells in body fluids. Cellular immunity relies on the action of T-cells. Both mechanisms are interconnected and activate each other.

Mechanisms of Acquired Immune Response

In Humoral Immunity, B-cells recognize antigens using surface receptors, internalize them, and present them on their surface bound to the Major Histocompatibility Complex ($MHC$). Simultaneously, macrophages engulf pathogens and present the antigen-$MHC$ complex. $T_H$ cells bind to this complex on macrophages, releasing interleukins that activate the B-cells. The activated B-cells divide and differentiate into plasma B-cells, which produce massive amounts of antibodies, and memory B-cells, which remain in the body for $20-30$ years.

In Cellular Immunity, activated $T_H$ cells secrete cytokines that attract more macrophages and stimulate B-cells, $T_C$ cells, and NK-cells. $T_C$ cells recognize foreign bodies through specific receptors and secrete Perforin to create pores in the infected cell's membrane, and lymphotoxins to activate genes that destroy the cell's nucleus, leading to its death.

After the pathogen is eliminated, the immune response must be inhibited. This is the role of $T_S$ cells, which secrete lymphokines to stop antibody production and cause the death of many activated $T_H$ and $T_C$ cells. Some cells are stored in lymphoid organs for future use.

Primary and Secondary Immune Responses

The primary immune response occurs during the first exposure to a new pathogen. It is a slow response, taking $5-10$ days for the concentration of antibodies to become effective. During this time, symptoms appear because the infection is widespread. Memory B and T cells are produced but remain inactive.

The secondary immune response occurs upon exposure to the same pathogen previously encountered. This response is very fast because the memory cells are already present in large numbers. Memory cells differentiate into plasma B-cells and activated T-cells almost immediately. As a result, the pathogen is destroyed quickly, and no symptoms appear. Memory cells can survive for decades, or even a lifetime, whereas standard B and T cells survive for only a few days.